1. Field of the Invention
This invention relates to a circuit for high frequency arc welding apparatus, and more particularly to a circuit in which the welding current is controlled by controlling the voltage of a condenser (or a D.C. electrical source) for regenerating the electrical energy stored in the inductance of the power circuit cables.
2. Description of the Prior Art
It is well known that as the high frequency pulse component of 10 KHz to 100 KHz involved in the welding current of a D.C. arc welding apparatus makes the stiffness and the stability of the arc increase, the welding characteristics can be improved and the welding speed accelerated.
The welding result is effected by the high frequency arc current as follows. The higher the frequency, the better the welding characteristics, and also the larger the ripple component involved, the better the welding characteristics.
A prior art high frequency arc welding apparatus is shown in FIGS. 1 and 2, in which FIG. 1 is the circuit diagram and FIG. 2 shows the waveform of the welding current obtained. In FIG. 1, 1 is a D.C. electrical source, 2 a cable, 3 a parent metal, 4 a transistor, 6 a cable, 7 an electrode, 8 arc, 9 a diode, 10 the control circuit for the transistor 4, 11 a resistor, 12 a transistor, 13 a resistor, 14 a current control circuit, 15 the control circuit for the transistor 12, 16 a diode, and 17 a shunt resistor for welding current detection. Initially, the transistors 14 and 12 are turned on at the same time by the control circuit 14 through the other control circuits 10 and 15. As the result, the welding current begins to flow as shown in FIG. 2 through the path 1-12-2-8-6-17-14-1. At the time t1, the transistor 4 is turned off by the control circuit 14, so that the current takes the path 12-2-8-6-17-9-12. Accordingly, the welding current gradually decreases with a small decelerating slope as shown in FIG. 2, because the cables 2 and 6 and other parts of the welding apparatus have some inevitable inductance. At the time t2, the transistor 12 is also turned off, so that the current takes the path 6-17-9-1-16-2. Accordingly, the welding current decreases rapidly as shown in FIG. 2, because the D.C. electrical source 1 is connected opposite to the current direction. During the operation of the apparatus, the frequency is generally kept constant.
It would seem that the welding characteristics would be better than in other conventional welding apparatuses, because the mean value of the welding current can be controlled by controlling the interval t1 to t2, keeping the peak value Ip (amplitude of the pulse component) of the welding current waveform constant. However, the operating efficiency and reliability of the apparatus are worse than for other conventional welding apparatuses because the two high frequency switches 4 and 12, which are complicated and have many parts, are connected in series to each other in the welding circuit.